This invention relates to a chemical method of decontaminating metallic wastes that have been contaminated by radioactive substances and which have resulted from the demolition of nuclear power plants or reactors in operation.
Metallic wastes contaminated with radioactive substances must be decontaminated to unrestricted levels before they are disposed of as non-radioactive wastes. To this end, many methods have been developed and two typical examples are an electro-polishing method in which an electric current is applied to the contaminated metallic waste to perform anodic dissolution and a mechanical method in which the metal surface is ground with a particulate abrasive. These methods have the advantage that satisfactory decontamination can be accomplished within a short period of time but on the other hand, their applicability is limited to metallic parts of simple shapes and it is usually difficult to decontaminate uniformly the surface of complexly shaped parts such as valves and pumps.
With a view to dealing with this problem, a chemical decontamination method has been proposed by which metal parts are decontaminated with solutions of various chemicals (i.e., decontaminates). A problem with this approach is that if a solution of an inorganic acid such as sulfuric acid or nitric acid is solely used as a decontaminant, the radioactive "crud" (radioactive corrosive product) on the metal surface as well as the base metal dissolve so slowly that it has been difficult to achieve a high decontamination factor (DF) within a short period of time.
In order to increase the speed at which the "crud" and the base metal dissolve, it has been proposed that a solution in which an oxidizer such as cerium (Ce.sup.4+) or potassium permanganate (KMnO.sub.4) is combined with an inorganic acid be used as a decontaminant. The present inventors previously developed a method that relied on this approach and achieved satisfactory decontamination by using sulfuric acid/cerium (SC) in solution (see Japanese Patent Public Disclosure Nos. 62-261099 and 63-235899). However, when the oxidizer (Ce.sup.4+) is used to oxidize and dissolve the "crud" and base metal (Fe.sup.2+ .fwdarw.Fe.sup.3+ +e-, Cr.sup.3+ .fwdarw.Cr.sup.VI +3e- and Fe.fwdarw.Fe.sup.3+ +3e-, Cr.fwdarw.Cr.sup.VI +6e-, etc.), it is reduced (Ce.sup.4+ +e-.fwdarw.Ce.sup.3+) to lose its oxidizing and dissolving ability. It is therefore necessary to either supply an additional amount of the oxidizer that compensates for its consumption or regenerate the corresponding amount of oxidizer by electrochemical means. Further, if the concentration of dissolved metallic ions exceeds 1.times.10.sup.4 ppm (1 kg/m.sup.3), an increased amount of electric current must be applied in order to compensate for the drop in current efficiency which occurs during regeneration of the oxidizer. In an extreme case, complete replacement of the decontaminant solution is necessary but this causes the disadvantage of generating large quantities of liquid wastes from the operations of decontamination.
The present inventor found that in a certain concentration range of sulfuric acid, carbon steels or stainless steels (e.g. SUS 304 and 316) were corroded more rapidly by aqueous sulfuric acid than by sulfuric acid/cerium (SC) in solution. The results of an experiment conducted in this regard are shown in FIG. 1. The experiment was conducted in order to investigate the relationship between the concentration of sulfuric acid and the rate of corrosion (mdd, mg/dm.sup.2.day) of stainless steels (SUS 304 and 316 ) and Inconel 600 (which are representative of the materials of which nuclear reactors are constructed) in aqueous sulfuric acid and sulfuric acid/cerium (SC) in solution. The corrosion test was performed by the following procedure: a predetermined concentration of aqueous sulfuric acid or SC in solution was placed in an amount of 700 ml in a 1-l glass separable flask and heated to 80.degree. C.; thereafter, a metal test piece having a surface area of 22.4 cm.sup.2 was immersed in the solution with stirring. The test piece had been polished with emery #400, degreased and cleansed. As is clear from FIG. 1, the rate of corrosion of the stainless steels (SUS 304 and 316) increased with the increasing concentration of sulfuric acid ( , ) but the rate of corrosion of Inconel 600 was little susceptible to the concentration of sulfuric acid ( ). In the SC solution, the corrosion rate increased in proportion to the first order of the concentration of Ce.sup.4+ but it was little susceptible to the concentration of sulfuric acid (o, .DELTA., .quadrature.,). It is therefore clear that a stainless steel used as a base metal is corroded more rapidly by aqueous sulfuric acid than the SC solution if the concentration of sulfuric acid is held higher than a certain level, which is 0.7M for SUS 304 and 2.2M for SUS 316. However, Inconel 600 cannot be corroded with aqueous sulfuric acid at a sufficiently high rate and hence must be decontaminated with the SC solution. It was therefore established that stainless steels could be corroded at high rate with aqueous sulfuric acid having concentrations higher than certain levels. A similar experiment was conducted with carbon steels and sufficiently high corrosion rates (ca. 1.times.10.sup.5 mdd) could be attained with 0.25M sulfuric acid at 80.degree. C. In fact, however, metals contaminated with "cruds" could be corroded at fast rate only in the initial period of immersion in aqueous sulfuric acid and as time passed, the rate of corrosion slowed down and no high DF could be attained within a short period. Upon repeated use of the aqueous sulfuric acid in decontamination work, its capability as a decontaminant was eventually lost.